In March 1933, A few days after his Inauguration as President, Franklin Roosevelt left the White House to pay his respects to 92-year-old former Supreme Court Justice Oliver Wendell Holmes Jr. The amiable Roosevelt and the dour Holmes chatted, and after F.D.R. left, Holmes supposedly remarked that the new President had a "second-class intellect but a first-class temperament." Many historians now believe that Holmes was talking about Teddy Roosevelt rather than Franklin, but the story is oft told because it suggests a larger truth: that the most important attribute of a President is not intellect but something both more familiar and...